Wednesday, September 05, 2012

Flash Flood Watches Continue Today

We're still under a Flash Flood Watch locally until 8 PM this evening. The surface front that will bring an end to the heavy rainfall threat is back across Western and Central Pennsylvania, with our region firmly in a tropical airmass as dew point values are in the middle 70's this morning.

The tropical moisture is not just at the surface but also through the lower portions of the atmosphere. This graphic below shows what was the PWAT (precipitable water) at 5 AM across the region. The sharp gradient across Central Pennsylvania is where the front is setting up. Our region was under the heaviest precipitable water values earlier -- the term basically means that if you were to squeeze out all of the water in the atmosphere over your head that 2.2" of water would fall out of the sky. These type of PWAT values are high for this region and suggest a strong tropical connection...which there is thanks to Isaac's circulation having pulled this moisture north over the past couple of days.

The result of this strong influx of tropical moisture has been the rainfall in our region being as robust as it has been between Monday and yesterday...and until the front crosses the region later today, rains at any time today will run the risk of being locally heavy and localized flooding is possible.

As with any tropical influence, there is a very slight risk of a "weak" tornado or downburst to occur as winds are changing somewhat with height ahead of the front. Odds of this are low but the strongest of shower or storm could produce results similar to that of Camden, Delaware on Monday or what probably occurred in Mount Ephraim yesterday (the Mount Ephraim damage will be surveyed today but from accounts it does look like a "weak" tornado touched down).